TOKYO, Japan (CNN) -- International donors meeting in Japan have pledged more than $5 billion in aid for Pakistan to bolster the country's economy and help it fight terror and Islamic radicalism, officials said.

Japan and the World Bank co-hosted the 2009 Pakistan Donors Conference, with representatives of 31 countries and 18 international organizations and agencies.

"The international community is facing profound economic challenges," Hirofumi Nakasone, Japan's foreign affairs minister, said at the opening session of the donors conference.

"At the same time it must tackle the acute problem of terrorism, which is occurring all around the world and threatens people's peaceful lifestyles and their happiness.

He said Pakistan plays a key role in international efforts to combat terrorism and its stability is crucial to the region, particularly neighboring Afghanistan.

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Spiraling unrest in Pakistan and the rise of Taliban extremists in the country's northwest have alarmed the international community, which has repeatedly urged Islamabad to curb militants operating in remote border regions.

Afghanistan, where an international military force is struggling against a resurgent Taliban offensive, says most of the rebel fighters are entering its territory across the porous border from Pakistan.

President Asif Ali Zardari welcomed the pledges.

"I assure that with the support of world, Pakistan will overcome its challenges and get rid of the menace of terrorism and militancy," he said, according to the official Associated Press of Pakistan news agency.

The United States said it was donating $1 billion over a two-year period as part of President Barack Obama's plan unveiled last month to defeat to defeat the Taliban and al Qaeda militants in Afghanistan and Pakistan.

Obama said the money, slated to help build schools, roads, and hospitals, was was an investment in America's future "because the security of America and Pakistan is shared."

U.S. Special Representative for Afghanistan and Pakistan Richard C. Holbrooke said Monday that the pledge, subject to congressional approval, "is a down payment on President Obama's commitment."

He said the United States "is the single largest bilateral donor to the Pakistani people," adding that the package would strengthen economic growth and stability through agriculture, education, health, poverty alleviation, and energy.

"As President Obama has made clear, the U.S. is committed to working with the government of Pakistan and the international community to assist the people of Pakistan," Holbrooke said.

"With help from Congress, we will help meet our commitment with an assistance package that helps address Pakistan's most urgent social, economic, and welfare needs."

According to APP, Friday's meeting also saw a pledge of $1 billion from Japan plus others including $700 million from Saudi Arabia, $330 million from Iran, the $300 million from the United Arab Emirates, and $100 million from Turkey.

Last month, U.S. President Barack Obama unveiled a plan to defeat the Taliban and al Qaeda militants in Afghanistan and Pakistan, calling on Congress to authorize $1.5 billion every year over the next five years.

He said the money, slated to help build schools, roads, and hospitals, was a "down payment" on America's future "because the security of America and Pakistan is shared."